U.S. patent application number 09/931188 was filed with the patent office on 2003-02-20 for method and apparatus for stabilizing laser wavelength.
Invention is credited to Broutin, Scott L., Plourde, James K., Przybylek, George J., Stayt, John W. JR..
Application Number | 20030035617 09/931188 |
Document ID | / |
Family ID | 25460352 |
Filed Date | 2003-02-20 |
United States Patent
Application |
20030035617 |
Kind Code |
A1 |
Plourde, James K. ; et
al. |
February 20, 2003 |
Method and apparatus for stabilizing laser wavelength
Abstract
Laser wavelength stabilization is achieved by locating a low
selectivity reference filter in a reference path of a laser beam.
The low selectivity reference filter, with a periodicity greater
than the control filter, is used together with a lookup table for
the reference filter to resolve the uncertainty associated with the
multivalued control filter. After the wavelength uncertainty for
the control filter is resolved, the laser beam is stabilized based
on the response of the control filter, as if the reference filter
was not present.
Inventors: |
Plourde, James K.;
(Allentown, PA) ; Stayt, John W. JR.;
(Schnecksville, PA) ; Broutin, Scott L.;
(Kutztown, PA) ; Przybylek, George J.;
(Douglasville, PA) |
Correspondence
Address: |
DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
2101 L STREET NW
WASHINGTON
DC
20037-1526
US
|
Family ID: |
25460352 |
Appl. No.: |
09/931188 |
Filed: |
August 17, 2001 |
Current U.S.
Class: |
385/24 ; 372/32;
385/27 |
Current CPC
Class: |
H01S 5/0617 20130101;
H01S 3/1305 20130101; H01S 3/131 20130101; H01S 5/0687 20130101;
H01S 5/125 20130101 |
Class at
Publication: |
385/24 ; 385/27;
372/32 |
International
Class: |
G02B 006/28; G02B
006/26; H01S 003/13 |
Claims
What is claimed as new and desired to be protected by Letters
Patent of the United States is:
1. A laser system comprising: a control path and a reference path;
a control filter for stabilizing a laser beam, said control filter
having a first periodicity, said control filter being located in
said control path; and a reference filter for determining an
operating point cycle of said control filter, said reference filter
having a second periodicity greater than said first periodicity,
said reference filter being located in said reference path.
2. The laser system of claim 1, further comprising a device for
determining the wavelength characteristics of light transmitted
along said reference path and said control path.
3. The laser system of claim 2, further comprising a controller for
comparing said wavelength characteristics.
4. The laser system of claim 3, further comprising a laser medium
for generating said laser beam, and a servo system connected to
said controller for controlling said laser medium.
5. The system of claim 1, wherein said filters include an
etalon.
6. The system of claim 1, wherein said reference filter has a lower
selectivity than said control filter.
7. A wavelength division multiplex communication system,
comprising: a control path and a reference path; a control filter
for stabilizing a laser beam, said control filter having a first
periodicity, said control filter being located in said control
path; and a reference filter for determining an operating point
cycle of said control filter, said reference filter having a second
periodicity greater than said first periodicity, said reference
filter being located in said reference path; and an optical
waveguide for transmitting said laser beam.
8. The system of claim 7, wherein said waveguide includes an
optical fiber.
9. The system of claim 8, further comprising a device for
generating said laser beam, and wherein said control filter is
located between said device and said reference filter.
10. A laser system comprising: a resonator for generating a laser
beam; a control filter for stabilizing said laser beam, said
control filter being located in a control path, and said control
filter having a first periodicity; and a reference filter for
determining an operating point cycle of said control filter, said
reference filter having a second periodicity greater than said
first periodicity.
11. The system of claim 10, further comprising a wavelength monitor
for generating an output, said monitor being located in said
reference path.
12. The system of claim 11, further comprising a laser medium
located in said resonator, and a controller for responding to said
output of said monitor to control said laser medium.
13. The system of claim 11, further comprising a laser medium
located in said resonator, and a servo system operatively connected
to a controller for controlling said laser medium.
14. The system of claim 10, wherein said reference filter includes
an etalon.
15. The system of claim 10, wherein said reference filter has a
lower selectivity than said control filter.
16. The system of claim 10, further comprising a beam splitter for
transmitting a portion of said laser beam along said control
path.
17. The device of claim 16, further comprising a beam splitter for
transmitting a portion of said laser beam along a reference
path.
18. The device of claim 16, wherein said beam splitter is located
between said resonator and said reference filter.
19. A method of stabilizing the wavelength of a laser beam
comprising the acts of: transmitting light through a control filter
and a reference filter, wherein said control filter has a first
periodicity and said reference filter has a second periodicity
greater than said first periodicity; measuring the wavelength
characteristics of light on a reference path associated with said
reference filter; determining an operating point cycle of said
control filter based on said measured wavelength characteristics;
and controlling a laser medium within said operating point
cycle.
20. The method of claim 19, wherein said reference filter has a
lower selectivity than said control filter.
21. The method of claim 20, further comprising the act of
transmitting said beam in a wavelength division multiplex
communication system.
Description
FIELD OF THE INVENTION
[0001] This invention relates to laser wavelength stabilization
and, more specifically, to a method and apparatus for stabilizing
laser wavelength utilizing a relatively low selectivity optical
filter located in a reference path of a multi-path wavelength
control system.
DISCUSSION OF THE RELATED ART
[0002] An optical wavelength division multiplex (WDM) system may be
used to transmit an increased quantity of information as compared
to a single optical channel system. A WDM system, however, requires
a stabilized light source. Stabilization may be needed to transmit
laser lights of multiple wavelengths within a relatively narrow
wavelength band, for example, at intervals of 1 nm or less.
Moreover, in optical information processing or optical measurement,
the wavelength stabilization of the laser light source is an
important parameter for enhancing the density of information and
improving the precision of measurement.
[0003] The emission wavelength of a laser light source can be
stabilized by employing an optical filter having a predetermined
wavelength transmission characteristic, and by detecting an error
from the desired emission wavelength. A control signal, based on
the emission wavelength error, is fed back to the laser light
source for compensation. It is known, for example, to stabilize the
emission wavelength of a laser light source by using a multi-layer
interference optical filter or a Fabry-Perot (FP) etalon.
[0004] An FP etalon may be constructed for example as a multi-layer
single cavity filter type, where an all-dielectric
mirror/spacer/mirror structure is deposited on a glass substrate.
Alternatively, a solid etalon type may be used, in which mirrors
are deposited on both sides of a glass spacer plate. Whichever
implementation is used, the intensity of the beam that is allowed
through the etalon is a function of the wavelength of the beam
incident upon it. This principal is illustrated in FIG. 1 to aid in
the understanding of the present invention.
[0005] Etalon 100 is a quartz disk having flat opposed parallel
surfaces 103, 105 separated by a distance "d". In FIG. 1, as light
beam 107 passes through the front surface 105 of the etalon 100, a
reflected portion 107' is reflected back toward the back surface
103. The reflected beam 107' is itself reflected at the back
surface 103 to form another reflection beam 107" which passes
through the front surface 105.
[0006] Multiple reflections continue and if the distance d is equal
to a half wavelength, or a multiple thereof, of the beam 107, then
all reflections passing through the front surface 105 will be in
phase with the original beam 107. This reinforces the transmitted
beam such that maximum intensity for that particular wavelength is
transmitted, with the intensity decreasing for longer and shorter
wavelengths. (FIG. 1 illustrates a situation where a laser light
beam is coincident (or parallel) with the optical axis of the
etalon 100. Although the reflection beams 107', 107" are shown
displaced from one another in FIG. 1, they actually may be all on
the same optical axis.)
[0007] An FP etalon, like other optical filters, is periodic
(multi-valued). It must either possess a certain selectivity to
meet the stability specifications or possess a certain thickness to
allow practical handling during manufacture. In addition, it is
desirable but not necessary to have the periodicity of the FP
etalon agree with the International Telecommunications Union (ITU)
which is now set at .DELTA.f=100 GHz but may be set at 50 GHz or
less in the future. The periodicity or multi-valued nature of the
FP etalon can be a problem since it is difficult to determine which
cycle corresponds to the desired operating point.
[0008] Hence, what is needed is a method to avoid the uncertainty
associated with the periodic nature of high selectivity FP etalons
or other periodic filters. There is a need in the art for an
improved wavelength stabilization system that employs one or more
such high selectivity filters.
SUMMARY OF THE INVENTION
[0009] A method and apparatus for laser wavelength stabilization is
disclosed where a low selectivity reference filter is located in a
reference path of a laser beam, and a multi-valued control filter
is located in a control path. The reference filter, with a
periodicity greater than the control filter, is used together with
a lookup table to resolve the uncertainty associated with the
control filter, and then the control filter is used to lock the
laser source on the desired wavelength. When a change request to
another wavelength is received, the reference filter lookup table
is consulted to determine what parameters are associated with that
wavelength and the operation is then set within the desired cycle
or period of the control filter.
[0010] The above advantages and features of the invention will be
more clearly understood from the following detailed description
which is provided in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 schematically illustrates the operation of an FP
etalon (an example of a multi-valued optical filter);
[0012] FIG. 2 is an illustration of a wavelength stabilized laser
system constructed according to the present invention;
[0013] FIG. 3 illustrates a method of utilizing a reference period
which is twice that of the control period;
[0014] FIG. 4 illustrates a method of utilizing a reference period
which is quadruple that of the control period; and
[0015] FIG. 5 illustrates another method of utilizing a reference
period which is twice that of the control period.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0016] Preferred embodiments of the present invention are
illustrated in FIGS. 2-5. Other embodiments may be utilized and
structural changes may be made without departing from the spirit or
scope of the present invention. Although the invention is described
with reference to a single reference filter, the invention is also
applicable to laser systems that have more than one reference
filter. Also, although the reference filter may be an integral part
of the laser system, the invention is also applicable to lasers
having a separate reference filter. Like items are referred to by
like reference numerals throughout the drawings.
[0017] FIG. 2 is a schematic illustration of a laser system 9. The
system 9 has a laser medium 1 and a light resonator 11. The light
resonator 11 includes a high reflection mirror 2 and an incomplete
reflection mirror 3. The light generated by the medium 1 is
amplified while being reflected within the resonator 11 numerous
times before exiting as laser beam 6. The illustrated laser system
9 also has a reference filter 5, a control filter 4, beam splitters
25a, 25b, a wavelength monitor system 7, a control system 8, and a
servo system 10 for controlling the laser medium 1. If the laser
medium 1 is a DBR laser, it can be operated by the application of
tuning currents. The laser system 9 may be connected by optical
fiber 14 (or some other waveguide) to receiver 16. The receiver 16
may include a photodiode, for example.
[0018] Reference filter 5 is preferably an FP etalon. Reference
filter 5 has a lower selectivity than the control filter 4.
Selectivity is defined herein as .DELTA. transmission/.DELTA.
wavelength. Also, reference filter 5 is fabricated to have a
periodicity greater than that of the control filter 4. Periodicity
is defined herein as the wavelength range between consecutive
maximum intensity peaks and is related to the free spectral range
(FSR).
[0019] For a given wavelength range, a multiple number of maximum
peaks associated with the transmission through the control filter 4
is observed for every maximum peak associated with the transmission
through reference filter 5. The transmission through reference
filter 5 may be utilized to determine the desired operating point
cycle of the control filter 4, as will be discussed further
below.
[0020] Referring still to FIG. 2, the wavelength of light on
reference path 12a (i.e., the wavelength of the beam that is
reflected by the second beam splitter 25b) is approximately
measured (as a function of its intensity) by wavelength monitor
system 7. This measured value is then input into control system 8.
Control system 8 may contain data (the so called "lookup table")
corresponding to the various optical characteristics of the
reference filter 5. The control system 8 causes a certain bias
voltage and temperature (determined from the lookup table) to be
applied to the laser medium 1 by servo system 10 to bring the
wavelength of the light transmitted along reference path 12a to
near a desired target. The control filter 4, which is highly
selective, is then used to bring the wavelength of the light on
control path 12b (i.e., the portion of the beam 6 reflected by the
first beam splitter 25a) to the desired value within predetermined
specifications. A feed-back loop based on the wavelength
characteristics of the light transmitted along control path 12b
assures that the laser beam 6 locks onto the desired
wavelength.
[0021] FIG. 3 shows the reference and control responses 113, 115 of
the reference and control filters 5, 4, respectively. That is, the
normalized intensity of the light transmitted along the reference
and control paths 12a, 12b is shown as a function of the wavelength
of the laser beam 6. The reference response 113 has a reference
period characterized by maximum peaks 13a, 13b. That is, the
intensity of the light on reference path 12a is at a maximum value
(normalized value=1.0) when the wavelength of the laser beam 6 is
.lambda..sub.1 or .lambda..sub.2.
[0022] The control response 115 has a control period characterized
by maximum peaks 15a, 15b, 15c. The intensity of the light on
control path 12b is at a maximum value (normalized value=1.0) when
the wavelength of the laser beam 6 is .lambda..sub.3,
.lambda..sub.4 or .lambda..sub.5. The period of the reference
response 113 (i.e., the difference in wavelength between adjacent
peaks 13a, 13b) is greater than the period of the control response
115. In FIG. 3, the period of the control response 115 is the
wavelength difference between adjacent control response peaks 15b,
15c, such that:
.lambda..sub.2-.lambda..sub.1<.lambda..sub.5'.lambda..-
sub.4.
[0023] Assume, for example, still referring to FIG. 3, that the
wavelength of the laser beam 6 immediately following a wavelength
change request is .lambda..sub.6 (i.e., at a wavelength between
adjacent control response peaks 15b, 15c). Assume also that the
desired operating wavelength for the beam 6 is .lambda..sub.5 (at
the third control response peak 15c in FIG. 3). In this example,
the operating parameters of the laser 1 (bias, temperature, etc.)
would be modified first as a function of the output of the
reference filter 5 (i.e., based on the reference response 113), to
move the laser beam wavelength from .lambda..sub.6 to
.lambda..sub.2 (i.e., to the reference response peak 13b known to
be in the vicinity of the desired wavelength .lambda..sub.5). Then,
feedback control of the laser source 1 based on the output of the
control filter 4 (i.e., following the control response 115) would
be used to move the beam wavelength from .lambda..sub.2 to
.lambda..sub.5 (i.e., to the desired wavelength at the third
control response peak 15c).
[0024] Note that, in the foregoing example, if the step of making
an initial, approximate laser wavelength adjustment based on the
reference response 113 were not available, then it would be
difficult for the system to distinguish between the desired
wavelength .lambda..sub.5 and the wavelength .lambda..sub.4 at the
adjacent control response peak 15b. In other words, if the
reference filter 5 were not employed, it would be difficult for the
system to decide whether to move from .lambda..sub.6 to
.lambda..sub.5 or from .lambda..sub.6 to .lambda..sub.4.
[0025] In FIG. 3, the reference period
(.lambda..sub.2-.lambda..sub.1) is twice as long as the control
period (.lambda..sub.5-.lambda..sub.4). This two-to-one period
relationship has the advantage of maximizing the amplitude
difference of the reference response 113 with respect to cycles of
the control response 115 that are adjacent (but not within) the
desired operating point cycle. The advantage can be illustrated as
follows: In the example shown in FIG. 3, the normalized amplitude
of the reference response 113 is almost 1.0 in the vicinity of the
third control response peak 15c, and is about 0.1 in the vicinity
of the adjacent, second control response peak 15b. The normalized
difference in the amplitude of the reference response 113 between
the second and third control response peaks 15b, 15c is almost 0.9
(1.0-0.1=0.9).
[0026] In the illustrated example, the second control response peak
15b is the one that needs to be distinguished from the third
control response peak 15c. Consequently, it is advantageous to have
a large amplitude difference in the reference response 113 between
the second and third control peaks 15b, 15c, so that the reference
response 113 can be used effectively to move the wavelength of the
laser beam 6 toward the approximate desired wavelength
(.lambda..sub.2) If the period of the reference response 113 were
greater than twice that of the control response 115, then, with all
other parameters being equal, the normalized amplitude of the
reference response 113 would be equal to 0.1 at some point to the
left of the second control response peak 15b (in FIG. 3). As a
result, the normalized amplitude of the reference response 113
would be greater than 0.1 at the second control response peak 15b,
and the difference in normalized amplitude between the second and
third response peaks 15b, 15c would be less than 0.9.
[0027] The desired operating point cycle is the wavelength range
within which locking control for the laser beam 6 can be
accomplished based on the control response 115. In the system
described above, the wavelength 115 of the laser beam 6 is first
moved into the desired operating point cycle by moving it based on
the amplitude of the light transmitted through the reference path
12a. In an alternative embodiment of the invention, the wavelength
can be moved into the desired operating point cycle by moving it to
a point where the reference and control responses 113, 115 have a
predetermined relationship. For example, the wavelength of the beam
6 could be moved into the desired operating point cycle by moving
it from .lambda..sub.6 to the nearest point where the reference and
control responses 113, 115 are both high. Once the beam wavelength
is moved into the desired operating point cycle, the control
response 115 can be used to lock the wavelength of the beam 6 onto
the desired wavelength .lambda..sub.5.
[0028] Once the beam 6 is locked onto the desired wavelength, the
characteristics of the laser medium 1 (bias, temperature, etc.) are
then adjusted or maintained over time based on the control response
115 within the desired operating point cycle, until another change
request is received. That is, in a preferred embodiment of the
invention, the reference response 113 is only used to guide the
laser beam wavelength to the desired operating point cycle of the
control filter, and only the more selective control filter 4 is
used then to lock and maintain the beam 6 on the desired
wavelength.
[0029] In the method shown in FIG. 4, the invention operates with a
reference period (the wavelength difference between adjacent peaks
17) which is quadruple that of the control period (the wavelength
difference between adjacent control response peaks 19a, 19b, 19c
and 19d). The control system 8 compares the responses 119, 117 of
control filter 4 and reference filter 5, respectively, to determine
the proper operating point cycle of the control filter 4. The
characteristics of laser medium 1 are then adjusted by the servo
system 10 for operating conditions as required (i.e. adjusted for
bias, temperature, etc.) based on the wavelength characteristics of
the light transmitted along the control path 12b.
[0030] FIG. 5 shows another way in which the present invention can
operate with a reference period which is twice that of the control
period. The maximum peaks of the reference response 121 are
designated by reference numerals 21a, 21b. The maximum peaks
(wavelengths of greatest transmission intensity) of the control
response 123 are designated by reference numerals 23a, 23b, 23c and
23d. In this embodiment, the signs of the slopes of the reference
response 121 can be used to discriminate between desired and
undesired wavelengths. For example, the positive slope of the
reference response 121 at a first wavelength .lambda..sub.10 can be
used to distinguish the first wavelength .lambda..sub.10 from
second and third wavelengths .lambda..sub.11, .lambda..sub.12 which
have negative slopes.
[0031] In this manner, if the desired operating wavelength is at
the third peak 23c of the control response 123, then the system can
determine that the first wavelength .lambda..sub.10 is in the
vicinity of the desired wavelength and the second and third
wavelengths .lambda..sub.11, .lambda..sub.12 are not near the
desired wavelength. That is, the system can determine that the
first wavelength .lambda..sub.10 is in the desired operating point
cycle of the control filter whereas the second and third
wavelengths .lambda..sub.11, .lambda..sub.12 are not in the desired
operating point cycle, even though the amplitudes of the first,
second and third wavelengths .lambda..sub.10-.lambda..sub.12 are
all about the same.
[0032] If the desired operating point cycle happens to fall near a
maxima or minima of the reference filter 5 (near where its sign
changes), then two reference filters with slightly different
characteristics may be employed with the appropriate one chosen.
The measured values are input into control system 8 which compares
the slopes of the reference response 121 to determine the proper
operating point cycle of the control filter 4. The characteristics
of the laser medium 1 are then adjusted by the servo system 10 for
operating conditions as required (i.e. adjusted for bias,
temperature, etc.) based on the wavelength transmission
characteristics of the control filter 4, as if the reference filter
5 were not present.
[0033] Hence, highly selective laser wavelength stabilization may
be achieved by a system that has a low selectivity reference filter
5 located in a reference path 12a. The low selectivity reference
filter 5, with a periodicity greater than the control filter 4, is
used together with a lookup table for the reference filter 5 to
resolve the uncertainty associated with the multi-valued control
filter 4. After the desired region of the control filter response
is identified, the lookup table is used to calculate a control
signal to be applied to the servo system 10 based on the control
filter response. Thus, when a change request to another wavelength
is received, the reference filter lookup table is consulted to
determine what parameters are associated with that wavelength and
the operation is then set within a cycle of the control filter
4.
[0034] Although the invention has been described above in
connection with exemplary embodiments, it is apparent that many
modifications and substitutions can be made without departing from
the spirit or scope of the invention. Accordingly, the invention is
not to be considered as limited by the foregoing description, but
is only limited by the scope of the appended claims.
* * * * *